What is the mechanism by which an INTRAPULPAL injection produces anesthesia?

Correct answer: A. Back-pressure — anesthetic forced under pressure into a confined pulp space

• A. CORRECT. Intrapulpal anesthesia works primarily through PRESSURE, not pharmacology. The pulp is enclosed in rigid dentine walls, so injecting into that confined space under firm BACK-PRESSURE compresses the nerve fibres directly and produces almost immediate profound anesthesia. Two practical consequences follow: the needle must WEDGE tightly in the access opening so the solution cannot escape — a loose fit gives no pressure and no anesthesia — and the onset is immediate but the DURATION IS SHORT, typically 15 to 20 minutes, so pulp extirpation must proceed at once.
• B. Diffusion to the apex is far too slow to explain the immediate effect, and would not require the characteristic pressure or the tight needle fit.
• C. The anesthetic does not meaningfully alkalinise inflamed tissue — the acidic environment of inflammation works AGAINST anesthetic efficacy, which is part of why supplemental techniques are needed.
• D. Vasoconstriction contributes to duration in conventional infiltration but does not explain the immediate onset of an intrapulpal injection, which works even with plain solution.

Note: Warn the patient: the injection itself is briefly but sharply painful because pressure is applied to inflamed tissue. Reach for intrapulpal only after supplemental intraligamentary or intraosseous techniques have failed, since it is the most uncomfortable of the supplemental options.